1. Field of the Invention
The present invention relates to the field of integrated circuit packaging. In particular, the present invention relates to thin quad flatpack packages for integrated circuit devices.
2. Background Art
An integrated circuit device dissipates power primarily in the form of heat. The temperature of the semiconductor substrate depends upon the heat-carrying characteristics of the integrated circuit components and the ambient temperature of the surrounding environment. As the temperature of the semiconductor substrate approaches the highest operating limit of the semiconductor device, the performance of the semiconductor device substantially degrades. Typical semiconductor devices have an ambient operating temperature (also referred to as Ta in this application) ranging from 0.degree. C. to 70.degree. C., although some devices have extended ambient operating temperature ranges, such as from -40.degree. C. to +85.degree. C., or from -55.degree. C. to +125.degree. C. High temperature operation of a semiconductor devices reduces its operating life span, causes it to run at lower speeds, and causes it to display other non-ideal operating characteristics.
High voltage, high current, and high frequency applications often require semiconductor devices to dissipate substantial amounts of power to perform useful functions. Power, dissipated in the form of heat, travels via conduction through the semiconductor substrate into the molding or case enclosing the device. Typically, the enclosure transfers heat directly or indirectly (i.e., by an external heat sink) to the surrounding air. As advances in integration techniques continue to reduce the physical size of semiconductor devices, the size of the integrated circuit packages will be correspondingly reduced. Smaller enclosures have less volume to absorb heat and less surface area to radiate heat away from the semiconductor device. Thus, high density integrated circuits require greater power dissipation ability than low density integrated circuits.
Many applications utilize semiconductor devices in high temperature environments. Military and automotive applications often require semiconductor devices to operate in extreme temperature environments, such as in the desert or within an engine compartment. Miniaturized computer devices may also require high temperature operation of semiconductor devices. Highly-integrated semiconductor devices are utilized to reduce the physical size of computers and other electronic equipment. Integrated circuits are arranged closely together, often near heat-producing power supplies and mass storage devices. Cooling devices, such as air blowers and refrigeration systems, are often omitted from electronic devices to reduce their size, power consumption, cost, and overall complexity. Miniaturization of electronic equipment requires small outline and small profile integrated circuit packages capable of dissipating large amounts of heat.
The heat carrying characteristics of a substance is called its thermal resistance (.theta.) and is measured in units of degrees Celsius of temperature rise per watt of power dissipated (.degree.C./W). The thermal resistance between a semiconductor substrate and the outside environment may be calculated by summing the thermal resistances of all the intervening structures. In a typical application, thermal resistance may be calculated between a semiconductor junction and the ambient air. The thermal resistance between the junction and the ambient air (.theta..sub.J-A) is determined by: EQU .theta..sub.J-A =.theta..sub.J-C +.theta..sub.C-A
where .theta..sub.J-C is the thermal resistance between the junction and the case or enclosure, and .theta..sub.C-A is the thermal resistance between the case or enclosure and the ambient air. The junction temperature (T.sub.J) is determined by: EQU T.sub.J =T.sub.A +(.theta..sub.J-A)*P
where Ta is the ambient temperature of the air and P is the power produced by the semiconductor device. Therefore, reducing the thermal resistance between the junction and the ambient air (.theta..sub.J-A) increases the amount of power that a particular semiconductor device can dissipate at a given ambient temperature.
FIG. 1 shows a typical prior art Thin Quad Flatpack Package (TQFP) 100. TQFP 100 consists of a molded plastic package 104, leadframe 102, semiconductor die 106, die attach epoxy 108, die pad 110, and bond wires 112. As seen in FIG. 1, TQFP 100 does not provide a conductive path for release of the heat energy created by die 106. As a result, TQFP 100 is not suitable for the present and future generation of semiconductor chips which operate at high speeds and power consumption rates. These chips can be best supported by using thermally enhanced Thin Quad Flatpack Packages (TQFP). The thermally enhanced TQFP's permit chip operation at high speeds and power dissipation rates.
Thus far, several thermally enhanced semiconductor packages have been developed. However, each one of these thermally enhanced packages has severe disadvantages that limit its ability to meet today's demands for high speed devices used in small form factor electronic systems. An example of one known thermally enhanced package is package 200 shown in FIG. 2. As with the typical TQFP 100 shown in FIG. 1, package 200 consists of leadframe 202, molded plastic package 204, bond wires 212, semiconductor die 206, thermally conductive and electrically insulative tape 208, and die pad 210. However, package 200 also includes a "Drop-In-Heat Spreader" 214 attached to the underside of die pad 210. The outer surfaces of legs 216 of "Drop-In-Heat Spreader" 214 are exposed to the ambient air. Thus, "Drop-In-Heat Spreader" 214 permits the heat energy created in die 206 to flow to the case (i.e. molded plastic package 204) and the ambient air.
However, "Drop-In-Heat Spreader" 214 has several disadvantages. First, the "Drop-In-Heat Spreader" is limited to thicker package technologies such as Metric Quad Flatpack (MQFP), Small Outline Integrated Circuit (SOIC), Dual-In-Line Package (DIP), and Plastic Leaded Chip Carrier (PLCC). "Drop-In-Heat Spreader" 214 cannot be used in low profile plastic packages such as the TQFP. This is because "Drop-In-Heat Spreader" 214 is mechanically attached to die pad 210 (without use of an adhesive tape). In other words, legs 216 (instead of an adhesive tape) ensure that "Drop-In-Heat Spreader" 214 is in contact with die pad 210. Legs 216 extend from the bottom of "Drop-In-Heat Spreader" 214 to the bottom surface of package 204. Thus, legs 216 significantly add to the height of package 204. The high profile of package 204 (typically about 2.7 millimeters) prevents use of the "Drop-In-Heat Spreader" technique in small form factor electronic systems such as small form factor disk drives (which require thicknesses of about 1.4 millimeters).
Second, legs 216 of "Drop-In-Heat Spreader" 214 reduce the reliability of "Drop-In-Heat Spreader" 214. The reason is that the mold compound used to form molded plastic package 204 has a different thermal expansion coefficient than legs 216. Since legs 216 extend to the surface of molded plastic package 204, the difference in the thermal expansion coefficients results in creation of gaps at the surface of molded plastic package 204. This in turn results in ingression of moisture into molded plastic package 200. Corrosion due to moisture is a serious threat to reliability of integrated circuits. Third, since "Drop-In-Heat Spreader" 214 is not attached to leadframe 202, package 200 does not have a high heat dissipation ability. This is because the exposed portions of a leadframe would provide a large surface for heat dissipation. In other words, the thermal resistivity of the large exposed surface of a leadframe is relatively small. Accordingly, the heat dissipation ability of package 200 is reduced since "Drop-In-Heat Spreader" 214 is not attached to leadframe 202.
Another example of known thermally enhanced packages is package 300 shown in FIG. 3. Package 300 utilizes a "batwing/crab" configuration. Package 300 consists of molded plastic package 304, leadframe 302, die pad 310, semiconductor die 306, and bond wires 312. In this configuration, one or more of the leads of a "bat wing" leadframe 302 are fused to the die pad to provide increased heat transfer. Thus, while leads 305 are utilized for connection to the circuitry on semiconductor die 306, leads 303 are not connected to the circuitry on die 306, and are fused to the die pad to provide increased heat transfer. The "batwing/crab" configuration has a severe disadvantage in that it can only be used if some leads or pins of the semiconductor package are not used. This requires a package to include more pins than that required for the operation of the semiconductor chip.
Another example of a known thermally enhanced semiconductor package is package 400 shown in FIG. 4. Package 400 consists of leadframe 402, molded plastic package 404, bond wires 412, semiconductor die 406, and thermally conductive and electrically insulative tape 408. Package 400 uses heat spreader 414 for thermal enhancement. Package 400 requires an expensive process for separately tooling leadframe 402. Leadframes typically used in integrated circuit packaging come with a die pad. This die pad (shown, for example, in FIGS. 1 and 2) has been removed in package 400. The removal of the die pad requires an additional step and thus adds to the cost of the integrated circuit package.
In integrated circuit packages, such those shown in FIGS. 1 and 2, the entire internal leadframe (including the die pad portion) is silver plated to provide good thermal conductivity between the semiconductor die and the leadframe die pad. Since package 400 does away with the leadframe die pad, the heat spreader itself is used as a die pad. Accordingly, the heat spreader itself has to be silver plated in order to maintain good thermal connection to the semiconductor die. Thus, package 400 requires an additional step of silver plating the heat spreader. This additional step of silver plating the heat spreader adds to the cost of the package. For the above reasons, the assembly cost for package 400 is approximately twice the cost of assembly of the typical package shown in FIG. 1.
Yet another example of a known thermally enhanced semiconductor package is package 500 shown in FIG. 5. Package 500 utilizes heat sink 516 which is attached to the top side of semiconductor die 506 through thermally conductive and electrically insulative tape 508. Leadframe 502 transfers electrical signals to and from die 506 through bond wires 512. The assembly is encapsulated in molded plastic package 504. The heat sink configuration used in package 500 has several disadvantages. Package 500 requires an expensive process for separately tooling leadframe 502. As stated above, conventional leadframes include a die pad. The die pad (shown, for example, in FIGS. 1 and 2) has been removed in package 500. The removal of the die pad requires an additional step and thus adds to the cost of the integrated circuit package.
Moreover, in integrated circuit packages, such those shown in FIGS. 1 and 2, the entire internal leadframe (including the die pad portion) is silver plated to provide good thermal conductivity between the semiconductor die and the leadframe die pad. Since package 500 does away with the leadframe die pad, the heat sink itself has to be silver plated in order to maintain good thermal connection to the semiconductor die. Thus, package 500 requires an additional step of silver plating the heat sink. This additional step of silver plating the heat sink adds to the cost of the package. For the above reasons, the assembly cost for package 500 is approximately three to five times the cost of assembly of the typical package shown in FIG. 1.
Another disadvantage of package 500 is the poor reliability of the package due to exposed heat sink 516. The reason for the poor reliability is that the mold compound used to form molded plastic package 504 has a different thermal expansion coefficient than heat sink 516. Since heat sink 516 extends to the surface of molded plastic package 504, the difference in the thermal expansion coefficients results in creation of gaps at the surface of molded plastic package 504. This in turn results in ingression of moisture into molded plastic package 500. Corrosion due to moisture is a serious threat to reliability of integrated circuits. One prior art solution is to plate heat sink 516 with corrosion-resistant material. However, this would add to the cost of manufacturing package 500.
Thus, there is need in the art for a thermally enhanced semiconductor package for small form factor electronics systems, which has good heat transfer capability and reliability, and does not require expensive tooling of the leadframe and heat spreader.